Acoustic wave filter with resistive termination, including side branch chamber



w. P. MASON 2,468,454

April 26, 1949.

. ACOUSTIC WAVE FILTER WITH RESISTIVE TERMINATION,

INCLUDING SIDE BRANCH CHAMBER .Filed May 10, 1944 lNl EN TOR By W P. MASON WFW Patented Apr. 26, 1949 ACOUSTIC WAVE FILTER WITH RESISTIVE TERMINATION, INCLUDING SIDE BRANCH CHAMBER Warren P. Mason, West Orange, N. J assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 10, 1944, Serial No. 534,913

2 Claims.

This invention relates to acoustic wave filters and more particularly to a special resistive termination for use with such a filter.

The principal object of the invention is to improve the performance of an acoustic wave filter which works into a load having a lower impedance than the image impedance of the filter. Another object is to provide for such a filter a termination which is mainly resistive and substantially equal to the image impedance of the filter over the major part of the transmission band. A further object is to provide an efiic'ient narrow band lowpass acoustic filter which will freely transmit a steady fiow of gas.

When a narrow band low-pass acoustic filter is to work into a low impedance load, such as the atmosphere, it is practically impossible to design a filter of reasonable size having an image impedance which is low enough to match the load. Such a filter may, for example, be used as a muffler to silence the exhaust of an internal com bustion engine. The filter must freely transmit a steady flow of gas but must attenuate down to a very low frequency. Due to this lack of a proper resistive termination for the filter in its transmission band, at certain engine speeds the frequencies corresponding to the natural resonances of the filter may be accentuated and the noise actually increased. Furthermore, with ordinary acoustic filters it is very dlfficult to provide attenuation at the low frequencies.

In accordance with the invention these difliculties are overcome by providing for the filter a special resistive termination which substantially matches its image impedance over the portion of the transmission band where these resonance effects occur but does not interfere appreciably with the steady flow of gas through the filter. Furthermore, the termination adds considerable loss in the low frequency range.

' In the preferred embodiment the termination comprises an outlet tube, a side branch and an acoustic resistance. The outlet tube communicates with the filter at one end, is open to the atmosphere at its other end, has a characteristic impedance approximately equal to the nominal image impedance R of the filter and is antiresonant at some frequency within the transmission band. The side branch has a characteristic impedance preferably smaller than R, is resonant at approximately the frequency f and is connected to the inner end of the outlet tube through the acoustic resistance which has a value approximately equal to R. The frequency f is preferably in the neighborhood of three-quarters of the cut off frequency of the filter. The exposed portions of the termination and the filter are preferably made of layers of sheet metal with an interposed layer of some sound absorbing material such as felt.

The nature of the invention will be more fully understood from the following detailed description and by reference to the accompanying drawings in which like reference characters refer to similar or corresponding parts and in which:

Fig. 1 is a longitudinal sectional view of an acoustic filter with a resistive termination in accordance with the invention;

Fig. 2 is a cross-sectional view taken along the line 2-2 of Fig. 1; and

Fig. 3 is a cross section of the central conduit of Fig. 1 taken at the line 3-3.

Taking up the figures in more detail, Figs. 1 and 2 show an acoustic wave filter 4 to which is attached a resistive termination 5 in accordance; with the invention. It is assumed that the sound: waves, for example, from the exhaust of an in ternal combustion engine, enter at the left, as; indicated by the arrow 6 and, after passing:

' be resonant at different frequencies and thereby provide separate peaks of attenuation. Each of the chambers 14 and I5 is connected to the conduit 8 through a number of narrow apertures l8 formed by cutting longitudinal circumferentially spaced slits in the conduit 8 and pressing out each alternate portion l9, as shown in Fig. 1 and in the cross section of Fig. 3. Forming the apertures ill in the manner shown helps to prevent a whistling elfect which might otherwise develop, especially if the filter is required to handle a large fiow of gas.

The filter is designed to have a nominal image impedance R and a cut-off at some desired frequency fc. In a low-pass filter the nominal image impedance is the image impedance at zero frequency. A discussion of the principles, and the formulas, applicable to the design of the filter may be found, for example, in United States Patents 1,692,317 issued November 20, 1928 and 1,874,326 issued August 30, 1932.

The resistive termination 5 comprises an outlet tube 29, a side branch 2| and an acoustic resistance 22. The left end of the outlet tube 20 is connected to the central conduit 8 and its other end 23 is open to the atmosphere. The tube 20 and the conduit 3 are preferably of substantially the same diameter and in axial alignment. The tube 20 is antiresonant at some frequency f within the transmission band of the filter 4 and has a. characteristic impedance approximately equal to the image impedance of the filter at that fre-. quency. The frequency f is preferably chosen as approximately three-fourths of the cut-off frequency. In order to make the outlet tube 20.anti-,

resonant at the frequency 1, its length, measured from the line 33 to the discharge end 23, is made equal to a quarter wavelength at that frequency. The length of the tube 20 may be shortened somewhat, but at a sacrifice in the efficiency, of the filter, by making it antiiesonant at a higher frequency, with the cut-if frequency as the upper limit.

The side branch 23 is connected at its upper end to the outlet tube 28 near the input end of the tube 23 and at its other end is closed by the plate 24. In order to make the structure more compact the branch 21 has a 90-degree bend at the point 25 and folds against the lower side of the filter 4. The lower portion 26 of the cylinder it! forms the upper wall of the branch 2|. The branch 2! is resonant at approximately the frequency and thereforehas a length equal tov a quarter wavelength at that frequency. Inorder to increase the attenuation of the filter in the low frequency range the characteristic impedance of the branch 2i should be low compared to R. Therefore, the cross-sectional area of the. branch 2i is made as large as is convenient. The side branch 2% is coupled to the outlet tube ZOthrough the acoustic resistance 22 which comprises the apertures 28,, the number and cross-sectional area of which are so chosen that it will have a value approximately equal to the image impedance of the filter 4 at the frequency f.

In order to prevent the radiation of acoustic energy due to the shock excitation of the structure by the exhaust from the engine, all of the exposed portions of the filter fl and the termination are preferably made of a highly damped material. As shown, these parts may be made of two thin layers 3!) and 3t of metal, such as sheet iron or steel, and an interposed layer 32 of sound absorbing material, such as felt.

When the termination is designed in the manner set forth herein, it will present to the filter 4 substantially a zero impedance at zero frequency and therefore will not impede a steady,

flow of gas through the system. However, as the frequency increases the impedance of the termination 5 develops a gradually increasingresistive component and at the frequency f this impedance is non-reactive and equal to theimage impedance of the filter, thus providing a perfect impedance match. Furthermore, over the major part of the transmission band the impedance of the termination 5 will be mainly resistive and sufficiently nearly equal to the image impedance of the filter to prevent appreciable reflection effects at the junction or the establishment of standing waves within the filter. Therefore, the accentuation of the natural resonant frequencies of the filter will be prevented.

Another advantage of the resistive termination 5 is that it adds loss at low frequencies where it is difficult to provide loss by other means. The impedance of the branch 2| is infinite at zero frequency and gradually falls to zero at the resonant frequency I. On the other hand, theimpedance of the outlet tube 20 is zero at Zero frequency and increases to an infinite value at the frequency f. Therefore, as the frequency increases from zero to f more and more energy is shunted through the branch 2|. Since the acoustic resistance 22 is effectively in series with the branch 2|v more and more of this energy is dis ipated in the resistance and prevented from reaching the atmosphere. At the frequency I this increase in loss may reach twenty decibels or more.

What is claimed is:

1,. In combination, a low-pass acoustic wave the nominal image impedance of said filter, and

said resistance having a value approximately equal to the image impedance of said filter at said frequency, whereby said filter is provided with a terminating impedance which substantially matches its image impedance over the major part of the transmission band.

2. The combination in accordance with, claim 1 in which the exposed portions of said filter, said outlet tube and said side branch comprise two layers of metal and an interposed layer of sound absorbing material.

WARREN P; MASON.

REFERENCES CITED Thev following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,243,866 Kamrath June 3, 1941 2, 97,0 6 Bourne Sept. 29, 1942 FOREIGN PATENTS Number Country Date 809,206 France Feb. 26, 1937 

